Chemical and biological sensing using metallic particles in amplifying and absorbing media

ABSTRACT

A film for surface enhanced raman scattering may be utilized for chemical and biological sensing. The film includes a polymeric layer, and a metallic nanoparticle having a cross-section, the metallic nanoparticle being embedded in the polymeric layer. The polymeric layer has a thickness less than a largest straight line through the cross-section of said metallic nanoparticle. The polymeric layer is selected from a group of absorbing media and amplifying media, and the metallic nanoparticle may be gold. The metallic nanoparticle may also be a shape selected from a group of spheroids and rods.

This is a continuation of prior application Ser. No. 11/452,454 filedJun. 13, 2006, the entire disclosure of which is incorporated byreference herein, which claims the benefit of Provisional ApplicationU.S. Ser. No. 60/689,850 filed Jun. 13, 2005.

BACKGROUND

Since the discovery of Surface Enhanced Raman Scattering (SERS) on roughsilver electrodes, a large volume of work has gone into enhancing thiseffect with the aim of developing ultra sensitive detection of chemicaland biological molecules. Of particular interest are chemical agentsused in warfare and biological molecules related to genomic applicationsand disease agents.

Several approaches to designing SERS substrates based around metallicnanoparticles and patterned surfaces have been developed. In all ofthese approaches, the basic guiding physics has been the use of plasmonresonance to enhance local fields along with charge transfer effectswhich enhance the matrix elements of the Raman process.

Recent work by Lawandy has shown that large field enhancements beyondthe conventional effect in non-resonant media such as liquids andtransparent solids can take place when the metallic particles (smallcompared to the wavelengths of interest) are placed in or nearamplifying media. In this work, the case of a plasmon resonance resonantwith the gain medium response was treated in the Drude Model limit ofthe metallic particle electronic response. It can also be shown that theuse of anisotropic metallic particles such as spheroids and rods ofvarying aspect ratios can be used to tune the required gain oramplification required to create the large external fields.

Subsequent work has shown that the amplifying medium effect is presentin cases of finite particle size and beyond the electrostatic limit ofthe particle modes. This basic effect of gain and localized plasmonexcitations can be further combined with the electromagnetic propertiesof arrays and photonic band gap structures to provide additional effectson the density of photon states and provide additional enhancements aswell as filtering effects useful for the development of chem-bio sensorsutilizing amplifying media to create gigantic molecular detectionsensitivities.

Further developments have shown that the nanoparticle plasmon resonanceson a passive substrate need not be resonant or overlap the gain orabsorption medium's resonance. FIGS. 1 a and 1 b show how the localsquare of the electric field just outside the particle surface isenhanced in the case of a particle surrounded by a dye film absorbingnear and at a frequency far away form the plasmon resonance for theparticles on the substrate alone (glass for example) respectfully.

When the film is thick enough (˜0.5 nm), the plasmon response and theexternal field are driven by the dielectric functions of the dye filmand not the substrate. This important fact means that the response takesplace at or near the absorbing or amplifying medium's resonances and notthe bare plasmon resonance. This in turn means that a number ofdifferent metallic particles can be used with virtually any gain orabsorption (or both) medium to tune the enhancement to where it isneeded for the specific application such as SERS. A large part of thisenhancement is due to a factor relating the external field to theinternal particle field which is inversely proportional to the highlydispersive absorbing/amplifying medium dielectric functions when thestrength of this response is sufficiently strong. Typically this occursin solid films of high density

FIG. 2 shows a silicon substrate with a random collection of goldnanoparticles prior to the deposition of an absorbing or amplifyingfilm.

The use of a thin (˜0.5 nm-10 nm) film of absorber around a metallicparticle results in dramatic enhancements in the field just outside thethin absorber layer. This enhancement is considerably larger than thatof the case of a nanoparticle surrounded by a shell of transparentmaterial with no sharp dispersions lines associated with the absorbingtransition. FIG. 3 shows the dielectric functions for a solid film ofdye coating the particles in FIG. 1. It is clear from these two figuresthat the enhancement occurs near the absorption resonances and inparticular near the dips in the real part of the absorbersusceptibility.

SUMMARY OF THE INVENTION

Provided herein is a film for surface enhanced raman scattering. Thefilm includes a polymeric layer, and a metallic nanoparticle having across-section, the metallic nanoparticle being embedded in the polymericlayer. The polymeric layer has a thickness less than a largest straightline through the cross-section of said metallic nanoparticle. Thepolymeric layer is selected from a group of absorbing media andamplifying media, and the metallic nanoparticle may be gold. Themetallic nanoparticle may also be a shape selected from a group ofspheroids and rods.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1 a and 1 b are graphical illustrations of absorbance versuswavelength for a particle surrounded by dye film.

FIG. 2 is a photograph of a silicon substrate with gold particles.

FIG. 3 shows the dielectric functions for a solid film of dye coatingthe particles in FIG. 1.

FIGS. 4 a-4 f show different geometries for attaching the chemical orbiological moieties of interest to the active SERS substrates described.

DETAILED DESCRIPTION OF THE INVENTION

An important feature of any SERS substrate is its ability to provideanalyte binding to specific chemical or biological moieties. In order torealize this while also creating an enhanced surface field due to thepresence of the amplifying or resonant absorbing medium surrounding themetallic particle or surface feature, we disclose the use of a substratecomprised of a substrate which is transparent or absorbing, a collectionof metallic nanoparticles, either random or ordered on the surface and afilm of absorbing or amplifying material whose thickness is less thanthe nanoparticle size.

The exposed metallic surface can be functionalized (e.g. thiol linkersfor Au particles) while the dye film allows for the amplifying effectdescribed to occur, boosting the local fields by orders of magnitude. Inaddition, the film will exhibit enhanced absorption at the pumpwavelength (ground state singlet absorption for a dye such as rhodamineof phthalocyanine or coumarin). It should be noted that enhanced SERS isalso expected purely from an absorbing medium surrounding the particleas shown in the data of FIG. 1. This effect is again due to the presenceof a strongly dispersive dielectric response of sufficient strength toprovide large fields at the surface of the particle.

Similar 3-D structures can be developed to create more surface area solong as there is sufficient gain and exposed areas of metal to affectbinding of the target molecules.

Several geometries are available for accessing the large local fieldswhich arise from either an absorber film tuned to the Raman pump orsignal or an amplifying film tuned to the Raman emission region. FIGS. 4a-4 f show different geometries for attaching the chemical or biologicalmoieties of interest to the active SERS substrates described.

FIGS. 4 a and 4 c show situations where a part of the metal (Au forexample) is available for the use of linkers to attach the molecules ofinterest (sarin, anthrax spores, DNA, proteins, etc).

FIG. 4 d shows the use of elongated nano-structures which can havedifferent enhancement factors for different polarizations and alsoexhibit lower electron damping when used with amplifying media. Thelatter case results in a lowering of the required gain to creategigantic local fields.

FIGS. 4 e and 4 f show a situation where the absorber or amplifyinglayer is used for attachment of the moieties of interest. Finally FIG. 4f shows the situation where a third thin film layer (<5 nm,functionalized SiO₂ for example) is used for binding and selectivity.

Sensor System Design

The substrates described can be used uses in compact detector systemswhich include spectral analysis of the SERS signals and spectralanalysis software. In the case of use with only a resonant absorberfilm, a diode laser source can be used for excitation. When theadditional gigantic enhancements achievable with a combination absorberand amplifying medium are used, there maybe a pump source for the systemto function. This source can be a number of intense sources includingpulses and Q switched lasers and in particular long life diode pumpedsolid state lasers including Raman shifters to access the requiredspectral bands for SERS. The substrate can be a transparent material toallow for pumping through the bottom of the structure or it can beabsorbing with a long pass behavior (semiconductor doped glasses) to beused as a filter for the pump radiation when the structure is pumpedfrom above.

While there have been described what are presently believed to be thepreferred embodiments of the invention, those skilled in the art willrealize that changes and modifications may be made thereto withoutdeparting from the spirit of the invention, and it is intended toinclude all such changes and modifications as fall within the true scopeof the invention.

1. A film for surface enhanced Raman scattering, comprising: a layerincluding a gain medium; and a nanoparticle capable of polaritonresonance, the nanoparticle having a cross-section and being embedded inthe layer; wherein the layer has a thickness less than a largeststraight line through the cross-section of the nanoparticle.
 2. The filmof claim 1 wherein the nanoparticle has a shape selected from a group ofspheroids and rods.
 3. The film of claim 1 wherein the polaritonresonance is plasmon-polariton resonance.
 4. The film of claim 1 whereinthe polariton resonance is phonon-polariton resonance.
 5. The film ofclaim 1 further comprising a substrate on which the layer is disposed.6. The film of claim 5 wherein the substrate is transparent.
 7. The filmof claim 5 wherein the substrate is absorbing.
 8. A film for surfaceenhanced Raman scattering, comprising: a first layer; a metallicnanoparticle having a cross-section and being embedded in the firstlayer; and a thin film layer disposed on the first layer for attachmentof moieties; wherein the first layer has a thickness less than a largeststraight line through the cross-section of the metallic nanoparticle. 9.The film of claim 8 wherein the first layer is selected from a group ofabsorbing media and amplifying media.
 10. The film of claim 8 whereinthe metallic nanoparticle has a shape selected from a group of spheroidsand rods.
 11. The film of claim 8 wherein the thin film layer comprisesfunctionalized SiO₂.
 12. The film of claim 8 wherein the thin film layerhas a thickness less than 5 nm.
 13. The film of claim 8 furthercomprising a substrate on which the first layer is disposed.
 14. Thefilm of claim 13 wherein the substrate is transparent.
 15. The film ofclaim 13 wherein the substrate is absorbing.